1. Field of the invention
The invention relates to the field of mass spectrometry sample analysis and it has more particularly as object the methods and devices for introducing a microflow of a sample whose ions are subjected to analysis into the mass spectrometer.
It finds a particularly important application in the field of isotopic analysis, which must often be carried out on small sized samples, for example because these samples are highly radioactive or particularly costly.
Prior Art
At the present time, mass spectrometers use either a thermo-ionic ion source, or an ion source with electronic bombardment of a gas flow.
The first solution has the advantage of allowing samples to be used of very small mass, frequently between 0.1 and 1 microgram. The sample, usually in liquid form, is deposited on a refractory metal ribbon. By evaporation of the liquid, a solid deposit is obtained. The ribbon is placed in the ion source of the apparatus, then heated to a high temperature (2600.degree. C. for example) by an electric current. The sample then emits neutral molecules and ions. These latter, accelerated and focused in the form of a particle beam, are subjected to the analysis.
Although this technique has the advantage of allowing very small amounts of samples to be used, it has on the other hand numerous drawbacks. The intensity of the ion currents obtained at the collector of the spectrometer is low: that implies in practice being able to measure currents as small as 10.sup.-17 amps, which requires multipliers. The emission of ions by thermoionization is not well known, its stability and evolution in time are not always perfectly mastered. It is necessary to take into account isotopic fractionation and mass discrimination effects by corrections, generally provided by calibrating the apparatus with known products. Stable and reproductible measurements can only be obtained by accurately controlling the purity of the sample, the method of its preparation and deposition, the purity of the refractory forming the support, the degassing, the temperature rise rate.
All these limitations mean that it cannot be expected to exceed an accuracy of about 1:1000 when the isotopic ratios in the sample are of the order of 1/200, which is current in the nuclear field.
In addition, a mass spectrometer using a thermo-ionic source cannot be used for conducting chemical composition analyses and it is difficult to connect it in line in a separation or processing line.
Some of the above-mentioned drawbacks of the thermo-ionic sources do not exist in electron bombardment sources, which are wider in their application since they allow not only chemical but also isotopic analyses, but imply that the sample to be analyzed is gaseous or readily vaporizable.
The usual method consists in introducing the sample from a sealed container through ducts and microleak valves letting through a well determined and very small gas flow as as not to alter the very low pressure which must reign in the analyser of the spectrometer. The gas or vapor molecules which pass in a very small amount are subjected to the action of an electron beam of predetermined energy which ionizes the gas so as to give rise to ions subjected to the analysis.
The intensity of the ionic currents obtained is usually of the order of 10.sup.-9 A, that is to say much higher than in thermo-ionization spectrometers, which simplifies the measurement.
Since it is possible to retain the vacuum in the source for introducing the sample, contrary to what happens in the case of a thermo-ionic source and since the time before stabilization is shorter, the time required for obtaining a result is reduced as a whole by a factor of about 4, which makes the apparatus usable in line.
As a counterpart to the above-mentioned advantages, electron bombardment sources have drawbacks which make them difficult to use in certain cases.
In particular, it is necessary to have samples of a larger size than in the first case and to handle them. It is in fact not known how to design sample bottles, transfer volumes and valves having internal capacities less than a few cubic centimeters. In addition, the interaction of the gas molecules with the walls which contain them causes memory phenomena to appear which influence the results of the measurements and require corrective factors to be taken into account, determined from consumable standards.
In other words, the measurements carried out by means of a spectrometer using an ion source by electron bombardment are as a general rule differential measurements which guarantee a high accuracy, typically 50 to 100 times higher than with a thermo-ionization source.
It can be seen that each of the known solutions has drawbacks which make it unsuitable for a number of applications. In particular, the ion sources using the ionization of a small molecular flow require large sample volumes, which represents a serious or even redhibitory constraint in some cases.